Implantable electrical stimulation devices have been developed for therapeutic treatment of a wide variety of diseases and disorders. For example, implantable cardioverter defibrillators (ICDs) have been used in the treatment of various cardiac conditions. Spinal cord stimulators (SCS), or dorsal column stimulators (DCS), have been used in the treatment of chronic pain disorders including failed back syndrome, complex regional pain syndrome, and peripheral neuropathy. Peripheral nerve stimulation (PNS) systems have been used in the treatment of chronic pain syndromes and other diseases and disorders. Functional electrical stimulation (FES) systems have been used to restore some functionality to otherwise paralyzed extremities in spinal cord injury patients.
Typical implantable electrical stimulation systems may include one or more programmable electrodes on a lead that are connected to an implantable pulse generator (IPG) that contains a power source and stimulation circuitry. However, these systems can be difficult and/or time consuming to implant, as the electrodes and the IPG are usually implanted in separate areas and therefore the lead must be tunneled through body tissue to connect the IPG to the electrodes. Also, leads are susceptible to mechanical damage over time, particularly as they are usually thin and long.
Recently, small implantable neural stimulator technology, i.e. microstimulators, having integral electrodes attached to the body of a stimulator has been developed to address the disadvantages described above. This technology allows the typical IPG, lead and electrodes described above to be replaced with a single integral device. Integration of the lead has several advantages including reduction of surgery time by eliminating, for example, the need for implanting the electrodes and IPG in separate places, the need for a device pocket, the need for tunneling to the electrode site, and requirements for strain relief ties on the lead itself. Reliability may therefore be increased significantly, especially in soft tissue and across joints because active components, such as lead wires, are now part of the rigid structure and are not subject to the mechanical damage due to repeated bending or flexing over time.
Unfortunately, the currently developed leadless devices tend to be larger and more massive than desirable, and even than traditional electrode/lead assemblies, making it difficult to stably position such devices in the proper position with respect to the nerve. Without device stability, the nerve and/or surrounding muscle or tissue can be damaged due to movement of the assembly. Further these devices require long charging times, and are often difficult to control (e.g., program) and regulate.
There remains a need for a leadless integral device that is stably positioned on the nerve, and can provide for removal and/or replacement of the stimulation device with relative ease.
In addition, prior art microstimulators have typically been designed to be injectable. Injectable stimulators rely not only on the accuracy of positioning, but the resulting arbitrary flows of current due to the in-situ heterogeneous tissues hosting the device. Current flows induced by non-shielded electrodes will vary in most situations with movements (e.g. neck movement, etc) making control of the relationship with the target (and therefore reliable stimulation of the target) all but impossible. In situations where the target thresholds are extremely low and the thresholds to invoke undesirable side effects are very high, an unshielded electrode from an injectable stimulator may be acceptable. However, for most situations, the target neuron is surrounded by tissues that are susceptible to unintended stimulation. While convenient to implant, these unshielded microstimulators are unacceptable for most applications. By containing the microstimulator in an isolated space with the nerve, the therapy window may be greatly increased.
Neurostimulator applications to this point have also been constrained to pulse generating applications where a constant pulse train is presented to the neural tissue. This pulse train may be gated or modulated to evoke the desired response and transfer the appropriate information to the target tissues. In other applications, such as FES, stimulation is presented to actuate muscles on demand. However, a neurostimulator may be used in an entirely new paradigm that is more like a pharmaceutical or biologic agent, where a dose is presented and a pharmacokinetic or pharmacodynamic response is elicited. This type of neural stimulation may result in a drug-like response delivered through a prescription written (e.g., “programmed”) by the administering physician. Delivering this type of stimulus typically requires short durations of stimulus followed by long durations of off-time, again typically administered in synchrony with the human circadian rhythm. Synchronizing such requires a sufficiently accurate real-time clock that initiates delivery of a stimulation dose.
Described herein are microstimulators and methods of using them that may address some of the needs identified above.